New US regulations on Thursday say that if you were exposed to COVID-19, you should take three home tests instead of two to make sure you’re not infected.
The Food and Drug administration had previously advised two rapid antigen tests over two or three days for accuracy. The agency now says that new studies suggest too many infections could be missed with only two tests, resulting in people spreading the coronavirus to others, especially if they are asymptomatic.
The new guidance applies to people without symptoms who may have been exposed. People with symptoms can continue using two tests spaced 48 hours apart.
This update reflects the changing understanding of antigen tests, which are not as sensitive as laboratory testing, but are often used due to their speed and convenience. These antigen tests detect protein traces, known as antigens, instead of the actual coronavirus. This is a similar method to flu testing.
Testing too early can give false negatives. The Centers for Disease Control and Prevention recommends people wait five days after an exposure, because antigens take several days to reach levels that nose swabs can test for.